Preheat oven to 350 degrees, with racks set in upper and lower thirds. In a large skillet, heat butter over medium. Add shallots, and cook, stirring occasionally, until softened, 1 to 2 minutes. Add as much spinach to skillet as will fit; season with salt and pepper, and toss, adding more spinach as room becomes available, until wilted, 2 to 3 minutes.

Transfer spinach mixture to a colander. Press firmly with the back of a spoon to squeeze out as much liquid as possible. Divide spinach mixture and cheese between prepared crusts. Place each crust on a separate rimmed baking sheet.

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